This invention relates to a plug and a plug set for use in a wellbore.
During the construction of oil and gas wells a borehole is drilled in the ground. A casing string is then lowered down the borehole and the annular space between the casing and the borehole filled with cement.
In order to facilitate cementation a float shoe and/or a float collar is inserted in or adjacent the bottom of the casing.
At the commencement of the cementation process fluid is pumped down the casing and allowed to flow through the annular space between the casing and the borehole. The fluid is introduced to remove any debris from the annular space and clean the outside of the casing. This process is often referred to as xe2x80x9ccirculationxe2x80x9d.
After circulation is complete, which typically takes several hours, a bottom plug is inserted into the casing and pumped down the casing by cement. After sufficient cement has been introduced into the casing a top plug is inserted into the casing and the column of cement, bounded by the top plug and the bottom plug, is pumped down the casing by drilling mud until the bottom plug lands on the float shoe. When the bottom plug lands on the float shoe the pressure on the top plug is increased until a diaphragm in the bottom plug ruptures thereby allowing the cement to pass through the float shoe and/or float collar and flow around the bottom of the casing and upwardly through the annular space between the casing and the wellbore.
When the top plug lands on the bottom plug the casing is immediately pressure tested by increasing the pressure of the drilling mud to the test pressure and, after stopping the pumps, checking to see whether any drop in pressure in a given time exceeds a design limit.
Once the test is completed the pressure inside the casing is relaxed and the cement (which is still wet) is allowed to set. The float shoe and/or float collar are essentially check vales which allow the flow of cement from the inside of the casing to the annular space between the casing and the borehole whilst inhibiting return flow therethrough.
After the cement has set the top plug, bottom plug and any cement set in the casing are drilled out before extending the borehole.
Conventionally, top plugs and bottom plugs have been extremely sturdy and massive construction.
One well accepted plug is described and shown in applicants"" PCT Patent Publication No. WO 94/15062. As can be seen from this publication the plug comprises a central core of hard, drillable polyurethane surrounded by a relatively flexible outer sheath having a plurality of fins extending therefrom. The top fin is extremely sturdy and is designed to withstand a pressure of typically 11,400 psi (780 bar).
The central core of the bottom plug is designed to withstand the pressure applied to the top plug.
Whilst these plugs have performed very acceptably they are not without their problems. In particular, they are relatively heavy and expensive. Furthermore, there is a tendency for the plugs to wobble as they travel downhole.
In the case of the bottom plug this can result in the bottom plug not landing properly on the float valve or collar. When the diaphragm bursts the overlap between the opening in the bottom plug and the opening in the float collar can reduct flow through the float valve thereby prolonging the cementing operation.
In the case of the top plug, if this does not land properly on the bottom plug and ends up skewed in the casing the upstream fin will not seal. This usually prevents the well being pressure tested since any attempt to pressurise the drilling mud above the top plug results in the drilling mud leaking, past the top plug, passing through the hole in the bottom plug and the float valve and entering fissures in the borehole or extruding the annulus of cement upwardly, which is highly undesirable.
Dealing firstly with the alignment problem, the tops of prior art plugs are generally substantially planar with the consequence that they are generally pushed down a wellbore from behind. The applicants believe that this is largely responsible for the wobble. A possible analogy is pushing a bicycle by holding the saddle. It will be appreciated that this is quite difficult as the front wheel tends to move away from the intended direction of travel, particularly if it encounters a stone or similar obstruction on the ground (corresponding to mud cake on the wall of casing). If the bicycle is steered from the front handlebars then the saddle follows.
By analogy, the Applicants submit that if a top plug is pushed from a position at or adjacent the downstream end thereof the upstream portion will follow and the chances of the plug being misaligned when it lands are reduced.
According to a first aspect of the present invention there is provided a plug for use in a wellbore, said plug comprising a body and at least a first fin which is made of a resilient material and which projects radially outwardly from said body to engage in use, the inner wall of a tubular, characterised in that said first fin is segmented and is provided with means to inhibit liquid passing through the gap between said segments.
In its simplest form said means may be provided by an extension piece integral with or attached to each segment. However, said means preferably comprises a second fin which is mounted downstream of said first fin and is segmented in such a way that, in use, said segments in said second fin engage said segments in said first fin and overlap the gaps therebetween.
If desired further segmented fins may be provided. Preferably, the segments of the first fin are mounted to pivot about a line which is disposed radially outwardly of a line about which the segments of said second fin pivots. This helps ensure that the segments of the second fin effectively engage the segments of the first fin.
Advantageously, the radial extremities of the segments of the second fin are provided with means which, in use, facilitate the removal of mud cake from the inside of casing. Such means may comprise, for example, ribs or protrusions on the radial extremity of at least some of the segments or even forming an angled surface on the radial extremity of the segments which acts like a chisel as the plug passes through a tubular.
By suitably dimensioning the segments it is conceivable that a very strong fin could be formed. However, in the context of the preferred embodiment it is anticipated that the fins will have a thickness of less than 3 mm.
Plugs in accordance with the present invention lend themselves to production in one piece particularly by injection moulding.
A suitable moulding material would be a polyamide, for example a polyamide currently sold under the trademark xe2x80x9c!!PETER!!xe2x80x9d by BASF of Germany.
It will be appreciated that a one piece injection moulding is far less expensive than the two stage moulding previously used in the production of conventional plugs.
The preferred embodiment satisfies certain needs which are now emerging. In particular, because of the greater depths of modern wells and the greater radial reach of many wells, it is becoming increasingly difficult to ensure that the cement is in a usable state by the time it reaches the bottom of the casing (or liner). In particular, it can take two hours for the bottom plug to travel from the surface to the float collar. If the bottom plug does not align properly with the float collar then the flow rate of the cement through the float collar can be restricted to such an extent that the cement passes through its xe2x80x9cfirst setxe2x80x9d before reaching the annulus.
High capacity float valves are being developed to reduce this problem. However, another solution is to use special chemical mixtures which precede and follow the slug of cement. These chemicals are strong solvents and tend to dissolve rubber which is most commonly used in the construction of plugs. Furthermore, such chemicals are particularly effective solvents at high temperatures.
Whilst the preferred materials of the present invention are, in their own right, resistant to chemical attack at high temperatures we have found that a plug having a body and fins formed in one piece by injection moulding has outstanding characteristics.
According to a second aspect of the present invention there is provided a plug for use in a wellbore, said plug having a body with an upstream end and a downstream end, characterised in that a major portion of the length of said body is hollow and open to the upstream end of said plug.
The plug may be a top plug, a bottom plug or an intermediate plug. In the case of the bottom plug or the intermediate plug a bursting disk or similar rupturable membrane will be provided at or adjacent the downstream end of the plug.
Advantageously, the major portion of the volume of said body is hollow and open to the upstream end of said plug.
It will be noted that by increasing the volume of the hollow portion of the plug the total amount of material required to manufacture the plug is reduced and the cost of manufacturing and transporting the plug reduced accordingly. However, using conventional concepts it is still important that the bottom plug should be able to support the top plug, the bodies of which are of equal diameter.
According to a third aspect of the present invention there is provided a plug set for use in a wellbore said plug set comprising a top plug and a bottom plug, characterised in that a major portion of the volume of said bottom plug is hollow and a major portion of the length of said top plug can be accommodated therein.
Advantageously, a major portion of the length of said top plug is hollow and open to the upstream end of said top plug.
Preferably, the downstream end of the bottom plug is provided with a nose which is preferably tapered and which can be received in a correspondingly tapered entrance to a float shoe or float collar. The nose is preferably provided with a seal to sealingly engage in the tapered entrance to the float valve.
In a similar manner the top plug is preferably provided with a tapered nose which can enter the tapered nose of the bottom plug and seat thereon. The tapered nose is preferably provided with a seal to facilitate the making of a seal between the tapered nose on the top plug and the tapered nose on the bottom plug.
By adopting this nesting construction the need for a strong core disappears. Typically, after a prior art cementing operation is complete and the cement set the top plug and the bottom plug are drilled out. This typically takes 15 minutes which is extremely expensive when it is recalled that a large offshore platform can cost over $1,000,000 a day to operate. In contrast, the nested top plug and bottom plug described above offer negligible resistance to the immense weight and power of a large drill which will simply pass through the nested top plug and bottom plug before drilling out the float collar and/or float shoe and any cement therebelow before drilling onwards.
The next area of investigation is the fin design. As indicated above the upstream fin on conventional plugs is usually massive. This is because the top of the top plug forms the platform on which the casing is pressure tested. The top plug itself must be supported by a substantial bottom plug which rests on the float collar or shoe.
If the upstream fin is not required to perform such an onerous duty then the design of the fin can be changed.
According to a fourth aspect of the present invention there is provided a plug for use in a wellbore, said plug comprising a body and fins, characterised in that said body and fins are formed in one piece by injection moulding.